1. Field of the Invention
The present invention relates to a semiconductor electroluminescent device. More particularly, the present invention relates to improvement in characteristics of a semiconductor electroluminescent device in a wide temperature range.
2. Description of the Related Art
In recent years, in the information communication field, for example, a semiconductor electroluminescent device which realizes both reduction in threshold current and high-speed operation in a wide temperature range (for example, −20° C. or higher and 95° C. or lower) is being developed.
A typical semiconductor electroluminescent device has, for example, a structure in which a lower light confinement layer, an active layer, an upper light confinement layer, and an InP layer are formed in the stated order on an InP substrate. Ordinarily, in the active layer, semiconductor thin films having different band gaps are alternately laminated. More specifically, the structure is a so-called single-quantum well (hereinafter, referred to as SQW) structure, or a so-called multiple-quantum well (hereinafter, referred to as MQW) structure. It is to be noted that the term “MQW” as referred to herein includes not only an ordinary MQW but also the SQW.
In the MQW structure, a semiconductor thin film having a large band gap serves as a barrier layer and a semiconductor film having a small band gap serves as a well layer. The barrier layer confines carriers within the well layer. Bonding of the carriers, i.e., electrons and holes, causes light emission.
InGaAsP-based and InGaAlAs-based materials are representative semiconductor materials used for an MQW layer in a semiconductor electroluminescent device. A main difference between the two materials is a band offset when the MQW structure is formed by the materials. The band offset is defined as an energy difference of a band edge of a barrier layer from a band edge of a well layer both in a conduction band and in a valence band. The former is represented as ΔEc and the latter is represented as ΔEv.
With regard to an InGaAsP-based material, ΔEc=0.4·ΔEg and ΔEv=0.6·ΔEg, while with regard to an InGaAlAs-based material, ΔEc=0.7·ΔEg and ΔEv=0.3·ΔEg. In order to obtain quantum effect, a thickness of a well layer in an MQW layer is ordinarily on the order of a Fermi wavelength with regard to both materials.
In a semiconductor electroluminescent device, in order to impose strain on a well layer, the composition of the well layer is ordinarily set such that the lattice constant thereof is different from the lattice constant of the substrate. This is because band degeneracy of light holes (hereinafter, referred to as LHs) and heavy holes (hereinafter, referred to as HHs) of the band structure at a band edge of a valence band of a well layer with no strain can be lifted by introducing strain in the well layer.
When the well layer has a lattice constant which is larger than that of the InP substrate, in other words, in the case of compressive strain, HHs contribute to oscillation, and considerable improvements in the characteristics of the electroluminescent device including the reduction in threshold current and the high-speed operation are realized. Similarly, when the well layer has a lattice constant which is smaller than that of the InP substrate, in other words, in the case of tensile strain, LHs contribute to oscillation (“Handotai Laser (Semiconductor Laser)” (Ohmsha, Ltd.), Chapter 5).